Transcript Chapter 11: Genetics

Chapter 11:
Genetics
When told to, put the PTC paper on your tongue
• PTC, or phenylthiourea, is an organic compound
having the unusual property of either tasting very
bitter, or being virtually tasteless, depending on the
genetic makeup of the taster.
• The ability to taste PTC is a dominant genetic trait.
• T = Taster t = non-taster
• If you can taste, you are either TT or Tt.
• Those who can not taste are tt
• About 70% of people can taste PTC, varying from a
low of 58% for Aborginal people of Australia to 98%
for Native Americans
• Ever wonder why people resemble their
parents &/or siblings?
• How do farmers select the best plants
or animals for breeding purposes?
• How’d you like that PTC paper?
• Why did some people in your class
taste it and others didn’t?
Genetics
• Studies the transmission of traits or
characteristics from 1 generation to the
next
Gregor Mendel
(1822 – 1884)
The Father of Genetics
• Central European monk discovered the
basic underlying principles of heredity.
• Work completed “Experiments on Plant
Hybridization” in 1865
• His work did not get recognized until
1900
• He used pea plants and selectively
crossed them over many
generations
• Saw that certain traits show up in
offspring plants without any blending
of parent characteristics.
X
Why peas?
1.Easy to grow in a short time
2. Ability to get pure breeds
3. Naturally self-pollinate (vs. cross-pollinate)
4. Isolated 7 different traits
White or purple flower color
Axial or terminal flower position
Long or short stems
Round or wrinkled seeds
Yellow or green seeds
Inflated or constricted
pods
Yellow or green pods
Mendel's work
1.
Removed the stamen (male parts) to prevent selfpollination
2. Used the stamen from a tall plant and pollinated only
flowers from other tall plants
3. Observed generations for 2 years to be certain of
purebreds
Tall
X
Tall
Short X Short
Yielded all Talls
Yielded all shorts
•
Crossed plants with contrasting traits
•
Tall
X
Short
Short X Tall
• P = Parental generation
• F1 = 1st Filial generation (offspring)
• F2 = 2nd Filial generation (next set of
offspring off the F1)
• P1: Pure Tall
X
Pure Short
T = Tall stems t = short stems
TT
X
tt
• F1:
Tall (Tt)
Only one trait showed
• P2:
• F2:
•
F1 Tall
X
F1 Tall
Tt
Tt
3 Tall
1 Short
TT Tt Tt
tt
The “lost” trait reappears!
• To get these results for
the F2, do FOIL – (firsts,
outers, inners, lasts)
• *******Always got 75%
Tall and 25% short or
3:1 whenever this type
of cross occurred.
Mendel’s 4 conclusions (hypotheses)
from his experiments
1. Concept of Unit Characters: The
inheritance of each trait is determined
by “units” or “factors” that are passed
on.
• We now know these “units” as GENES
• These “units” occur in pairs. One from
each parent
2. Law of Dominance: –
One factor “masks” the appearance of
another factor. It prevents it from
showing
• Dominant – trait that must show if
present (TT or Tt)
• Recessive – trait that will only show if
in the pure form (tt)
• Hybrid – Contains a dominant & a
recessive trait (Tt)
• Allele – alternate genes
for a given trait
(Tall or short)
3. Law of Segregation
• For any trait, pairs of alleles are separated
in forming the gametes.
• Only 1 gene (allele) from the pair goes
into the gamete during meiosis.
4. Law of Independent Assortment
• Different pairs of alleles are passed to
offspring independent of each other.
B = Brown
b = White
S = Short tail
s = Long tail
More genetic vocabulary to learn
• Genotype: Shows actual genetic
makeup. (Use symbols for genes)
• TT or Pure dominant or homozygous
dominant
• Tt or Hybrid or heterozygous
• tt or Pure recessive or homozygous
recessive
• Homozygous vs Heterozygous: Pure vs
Hybrid (mixed)
• Phenotype:
• Tells appearance (describes the trait)
Tall (if TT or Tt)
Short (if tt)
• NO HYBRIDS HERE!!
• Describes what you look like
• For Eye color, Blue, Green or Brown
• For Hair color, Red, Blond or Brown
Punnett Squares
•
•
•
•
Shows possible gene pairing & probability of each pairing.
Checkerboard method
Foiling
There are 2 kinds of sperm &/or 2 kinds of eggs depending on
the genes that they carry.
T
t
t
T
Tt
Tt
Tt
Tt
genes of 1 parent
genes of 1 parent
Phenotype ratio : 100% tall
Genotype ratio : 100% Tt (hybrid)
Possible crosses
• Do the crosses (Punnett Sqs) & determine
the :
•
Phenotype and Genotype ratios
•
Trait: T = Tall stem length t = short stem
length
•
1. TT x TT
•
2. TT x Tt
•
3. TT x tt
•
4. Tt x Tt
•
5. Tt x tt
•
6. tt x tt
Probability
• Likelihood of an event occurring
• Shown by Punnett Square
• Shows how often a gene pairing may occur.
**Need large numbers to get accurate
predictions**
Coin Toss Lab:
Flip one coin 10x. Keep track of the # of
Heads and Tails you get.
50:50 chance of getting heads or tail if you toss
one coin
What happens when you toss two coins?
• If you toss 2 coins 100 times, you should get:
•
25 Heads/Heads
•
50 Heads/Tails
•
25 Tails/Tails
• As the # of trials increases, the ratios
predicted by the laws of probability get
closer
• Actual outcomes get closer to calculated
predictions
Group
Class Total/100
Heads/Heads
Heads/Tails
Tails/Tails
So you are a right handed person. How do you know if
you are pure or a hybrid for the right handed trait?
Could you ever have a lefty child?
• Test Cross:
•
Method used to find out if something
(or someone) is pure dominant or a
hybrid for a given trait.
•
Cross them with a pure recessive for
that trait. If the results come up with a
recessive individual, then the parent
was a hybrid
• In Guinea pigs: B = Black coat
b = white coat
• Cross a BB guinea pig with a pure
recessive (bb)
• Cross a Bb guinea pig with a pure
recessive (bb)
• BB x bb
Bb x bb
Monohybrid crosses:
• Cross two hybrids. Bb x Bb
• Always get a 3:1 phenotypic ratio!!!
What are your chances of having a
blue eyed, blond haired child if
one of your parents is brown
haired and eyed and the other is a
blond with blue eyes?
Dihybrid crosses
• Crossing two hybrids, that are hybrid for two
different traits
• T = Tall stem
Y = Yellow seed
• t = short stem
y = green seed
•
TtYy x
TtYy
• Dihybrid cross
• Phenotype ratio for a dihybrid is always:
– 9:3:3:1 = (3:1)(3:1)
9 – Tall Yellow
3 – Tall green
3 – Short yellow
1 – Short green
• Trihybrid would be: 27:9:9:3:9:3:3:1 (3:1)(3:1)(3:1)